Acidic compounds, such as CO2, H2S and COS in acid gas streams can be separated or removed from the streams by scrubbing with a liquid absorbent medium. The scrubbing process is extensively used in petroleum production operations and in petrochemical refining processes. A number of different technologies are available for removing acid gases such as carbon dioxide, hydrogen sulfide, carbonyl sulfide. These processes include, for example, chemical absorption (amine), physical absorption, cryogenic distillation (Ryan Holmes process), and membrane system separation. Of these, amine separation is a highly developed technology with a number of competing processes in hand using various amine sorbents such as monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanolamine (MDEA), diisopropylamine (DIPA), diglycolamine (DGA), 2-amino-2-methyl-1-propanol (AMP) and piperazine (PZ). Of these, MEA, DEA, and MDEA are the ones most commonly used. The alkanolamine absorbents are usually preferred in view of the greater degree of solubility of the reaction products formed in the absorption process in water.
The amine purification process usually contacts the gas mixture countercurrently with an aqueous solution of the amine in an absorber tower. The liquid amine stream is then regenerated by desorption of the absorbed gases in a separate tower with the regenerated amine and the desorbed gases leaving the tower as separate streams. The various gas purification processes which are available are described, for example, in Gas Purification, Fifth Ed., Kohl and Neilsen, Gulf Publishing Company, 1997, ISBN-13: 978-0-88415-220-0.
An operational problem often encountered with acid gas abatement processes is corrosion of carbon steel and other low-alloy steels that are used in the construction of the piping and vessels of the treatment unit. The corrosion can be attributable to one or more of the following: decomposition of the scrubbing agent solvent, reaction of the acidic components of the gas and the scrubbing agent solvent; and direct attack by the acidic components in the gases. Another problem is the accumulation of heat stable salts (HSS) formed, for example, by the ingress of reactive contaminants and by degradation of amine, which in itself can lead to higher corrosion rate and equipment damage. HSS accumulation also consumes scrubbing agent solvents, reducing the amount available for acid gas absorption. Management of HSS levels can be achieved through a reclamation process or replacement of part of the amine inventory with fresh, uncontaminated amine (“bleed and feed” or larger bulk replacement) but corrosion inhibition technology has not been standard practice in amine treating systems due to the high cost of most programs and undesirable side effects (e.g., foaming). As such, for a typical acid gas treating facility, e.g., an amine treatment facility, the primary mitigation strategy for corrosion of carbon steel or other low-alloy steels has been to replace these materials with more corrosion-resistant stainless steel.
A more recent corrosion control program considered for acid-gas treating facilities involves the addition of soluble sodium tetrasulfide to the circulating amine. This program was shown in lab and field studies to form protective iron sulfide layers on carbon steel to reduce corrosion rates. Unfortunately, the high cost of the additive makes this concept economically infeasible. As an alternative, the introduction of soluble polysulfide ions was determined to be an effective corrosion mitigation strategy as they are effective to form protective sulfides on the walls of the equipment. U.S. Pat. Nos. 4,944,917, 4,857,283 and EP 102 712 describe the addition of ammonium or metal polysulfides or other means of forming polysulfide ions into the circulating amine treating solution. Although theoretically successful, the cost of these various chemical addition techniques has proved to be prohibitive in relation to their benefits and has resulted in limited commercial applicability to date. Alternatively, polysulfide ions can be obtained by the air oxidation of sulfide ions that are formed from dissociated hydrogen sulfide in the circulating amine solutions but air oxidation of sulfide ions degrades amine scrubbing agents, produces excessive quantities of additional oxidative HSS by-products that are detrimental to the process, and can react with diolefins to form a polymeric product that fouls equipment.
While the performance benefits of polysulfide ions have been determined, a more cost-effective method of generating the polysulfide ions is desired without the expense of, for example, ammonium or metal polysulfides additives, and without the disadvantages of obtaining polysulfide ions from air oxidized sulfide ions obtained from dissociated hydrogen sulfide. There also remains a need to integrate the polysulfide generation with effective management of corrosion of metal surfaces in acid gas removal processes used in petroleum production and refining operations as well as in chemical or petrochemical operations.
U.S. application Ser. No. 13/568,561, filed 7 Aug. 2012 discloses a method for operating an acid gas treatment unit with reduced corrosion in low alloy (carbon) steel treatment units utilizing the in situ generation of polysulfide ions by the use of an aqueous alkaline absorbent solution to absorb the H2S into the solution which is then contacted with elemental sulfur which reacts with the HS− and/or S2− ions to generate polysulfide ions.